Waterproof and anticorrosive fluorine-containing ferroelectric electret and preparation method thereof

Abstract

The application discloses a waterproof and corrosion-resistant fluorine-containing ferroelectric electret and a preparation method thereof, and belongs to the technical field of cross technology of functional polymer materials and flexible electronics. The fluorine-containing ferroelectric electret is prepared by edge fusion encapsulation of upper and lower dense polymer films and an intermediate layer, and then subjected to corona polarization treatment, and the intermediate layer is a single-layer structure of a porous polymer film or an alternating arrangement structure of a porous polymer film and a dense polymer film. The application provides structural regulation and edge fusion composite process innovation for the open-pore fluorine-containing multilayer polymer film, so that the prepared fluorine-containing ferroelectric electret not only has flexibility, high-temperature resistance and excellent piezoelectricity (greater than or equal to 400 pC / N), but also has waterproof and corrosion-resistant capability. After electrodes are connected to the opposite sides of the electret, the fluorine-containing ferroelectric electret is suitable for sensing, acoustics and energy collection scenes in harsh environments such as high temperature and salt water corrosion.

Description

Technical Field

[0001] This invention relates to the field of interdisciplinary technology of functional polymer materials and flexible electronics, and in particular to a waterproof and corrosion-resistant fluorinated ferroelectric electret and its preparation method. Background Technology

[0002] Ferroelectric electrets are a new type of piezoelectric material formed by separating and storing opposite charges on both sides of a polymer film with a porous internal structure under micro-plasma discharge conditions. Due to its flexibility, long-term stability, biocompatibility, and excellent piezoelectric properties, this material has become an ideal choice for wearable flexible electronic devices such as flexible energy harvesters, sensors, transducers, and acoustic devices.

[0003] Polypropylene (PP) ferroelectric electrets were developed in the late 20th century by Kirjavaine et al. in Finland. Their piezoelectric coefficient can reach 400 pC / N, but they require an operating temperature below 60°C and exhibit poor thermal stability. Fluoropolymers such as polytetrafluoroethylene (PTFE) and its copolymer fluorinated ethylene propylene (FEP) are widely used in electret material research and applications due to their high thermal stability and strong charge retention capabilities. Literature indicates that FEP can maintain a high surface potential in environments above 100°C, exhibiting excellent high-temperature electret performance (Remke and von Seggern, 2006).

[0004] Expanded PTFE (ePTFE) is a porous material produced through the stretching or expansion process of PTFE rods. It possesses good flexibility, adjustable porosity, and a high melting point. Studies have shown that ePTFE can exhibit a significant piezoelectric response after polarization. d 33 It can reach 400–500 pC / N (Hu et al., 2021). However, due to its open-pore structure, the internal channels are interconnected, and the space charge generated by polarization is prone to leak along the pore path, resulting in a short charge retention time (Zhukov et al., 2020).

[0005] To improve charge stability, some studies have employed a sandwich structure (FEP–ePTFE–FEP) sandwiching ePTFE between two FEP films, utilizing FEP's low conductivity and strong sealing ability to prevent charge leakage. This structure, after hot pressing and corona polarization treatment, exhibits good piezoelectric properties. Experimental results show… d 33 Approaching or exceeding 400 pC / N (Hu et al., 2021; Wang et al., 2021). Furthermore, the structure operated stably at 90°C for over 9 months, demonstrating good heat resistance and long-term charge retention.

[0006] Although the FEP–ePTFE–FEP structure effectively prevents charge leakage in the vertical direction, the edge regions of ePTFE remain open porous structures, easily becoming entry pathways for water vapor or impurity ions, affecting the device's charge retention capability and long-term stability (Small, 2021; Advanced Materials, 2024). Therefore, in practical applications, it is necessary to take sealing measures such as heat sealing and hermetical sealing at the structural edges to improve its waterproof and corrosion-resistant capabilities and enhance the device's performance in complex environments.

[0007] In summary, the combination of FEP and ePTFE can achieve a good balance between flexibility, thermal stability and electret performance, making it suitable for flexible sensing, acoustic devices and other applications. However, in order to improve its waterproof and corrosion-resistant capabilities, the structural design still needs further optimization to address the issue of edge openings.

[0008] A University of Connecticut team (2021, Small) developed an all-organic flexible ferroelectric electret nanogenerator with fabric-based electrodes for self-powered body area networks, based on ePTFE and conductive fabric, for harvesting energy from human movement, but did not discuss or specifically test its waterproof performance. Hang Hu et al. (2021, Waterproof and low-cost piezoelectrets with high piezoelectric responses) developed a polypropylene / polyethylene composite electret, sealing the air cavity with a polyethylene layer and a high-temperature resin framework, showing no performance degradation after 15 hours of immersion in water. However, the high hardness of PE and the presence of the resin framework inside the sample limited its deformation capabilities. Yunqi Cao et al. (Enabling Negative Pressure Sensing Through Ferroelectret Device) used polydimethylsiloxane (PDMS) as a waterproof coating for the polypropylene ferroelectric electret, but its dielectric properties and interfacial bonding with the substrate material (polypropylene) were not ideal, potentially leading to interfacial charge loss (especially under high humidity). Patent CN113497179A proposes a water-blocking layer composed of organic layers (such as silanes, siloxanes, and resins) and inorganic layers (such as oxides and nitrides) to provide a certain degree of waterproofing for ordinary electret films. However, the process is relatively complex and its resistance to high temperatures, chemical corrosion, and flexibility is relatively poor. Some studies have also used silicone to waterproof ferroelectric electrets, but silicone is prone to aging at high temperatures, limiting its application scenarios. Patents CN103474241A and CN115678360A employ coating methods, utilizing different technical solutions to improve the charge retention rate of ordinary electrets in high-temperature environments, but their waterproof and corrosion-resistant performance is poor, and the process requirements are strict and complex. Patent CN117309201A provides a biodegradable ferroelectric electret pressure sensor that uses polylactic acid (PLA) plastic wrap as an encapsulation layer to provide basic hydrophobic barrier. However, the biodegradable metal electrodes (such as magnesium and zinc) used in this sensor need to be actively corroded in body fluids, which conflicts with the need for corrosion protection. Furthermore, the porous electret layer with its corrugated structure is fixed by chitin sutures, which allows moisture to easily penetrate from the gaps.Patent CN1845353A describes a high-thermal-stability ferroelectric electret fabricated using a composite film system of polytetrafluoroethylene / fluorinated ethylene propylene copolymer (PTFE / FEP) and a grid pressing process. While achieving high thermal stability and piezoelectric activity due to material and process characteristics, it retains an open-pore structure with interconnected pores. In water mist environments, moisture can directly reach the charge retention surface, causing performance loss. Currently, there is limited research or patents on waterproofing and corrosion resistance for ferroelectric electrets. Some waterproof ferroelectric electrets often require a two-step hot-pressing process to create a PDMS film encapsulation, significantly increasing process complexity and cost. Existing ferroelectric electret materials still need further improvement in waterproofing and corrosion resistance to meet the growing demands of harsh environmental applications.

[0009] In summary, developing a novel ferroelectric electret material that retains excellent piezoelectric properties while possessing outstanding waterproof and corrosion-resistant capabilities as well as good high-temperature resistance is a significant challenge currently facing ferroelectric electret research. Summary of the Invention

[0010] The purpose of this invention is to provide a waterproof and corrosion-resistant fluorinated ferroelectric electret and its preparation method, to solve the problem of poor waterproof and corrosion-resistant performance of existing ferroelectric electrets in complex environments such as humidity and high temperature, while maintaining their piezoelectric properties. This technology achieves a synergistic improvement in piezoelectric and waterproof / corrosion-resistant properties through structural control of the open-pore fluorinated multilayer polymer film and innovative edge-melting composite process. It is suitable for flexible sensors in extreme environments (such as marine monitoring and industrial corrosive environments), self-powered energy harvesting devices (such as wearable devices and IoT nodes), and electronic monitoring systems under extreme conditions (such as new energy vehicle battery packs and aerospace equipment).

[0011] To achieve the above objectives, this invention discloses a method for preparing a waterproof and corrosion-resistant fluorine-containing ferroelectric electret, which is prepared by edge-melting and encapsulating two dense polymer films and an intermediate layer, followed by polarization treatment. The intermediate layer is a single-layer structure of porous polymer film or a multi-layer alternating structure of porous polymer film and dense polymer film.

[0012] Preferably, the porous polymer film has an interconnected and interwoven polymer filament structure, a thickness of 1-200 micrometers, and an opening pore size of 0.5-20 micrometers. The porous polymer film is any one of expanded polytetrafluoroethylene, porous polytetrafluoroethylene, porous perfluoroethylene propylene film, etc.

[0013] The dense polymer film is a fluoropolymer solid film layer.

[0014] Preferably, the fluorine-containing iron electret has a three-layer structure of FEP / ePTFE / FEP or a five-layer structure of FEP / ePTFE / FEP / ePTFE / FEP.

[0015] Preferably, edge melting encapsulation is achieved through a hot pressing process, including the following steps: stacking a porous polymer film as an intermediate layer on a hot press, adjusting the hot pressing parameters to melt and bond the edges of the upper and lower dense polymer films to form a seal.

[0016] In the above hot-pressing process, when the size of the intermediate porous polymer layer is smaller than the size of the dense polymer film, the intermediate layer is placed centrally between the dense polymer films. The edges of the dense polymer film overflowing from the intermediate layer are then melted and sealed. The hot-pressing temperature is 250–300°C, the hot-pressing pressure is 0.5–5 MPa, and the hot-pressing time is 30–120 minutes. Preferably, when the intermediate layer thickness is relatively large, typically 20 μm, a thin metal plate frame is placed at the edge of the top dense polymer film during the hot-pressing process. The thickness of the metal plate frame is consistent with the thickness of the intermediate porous layer after hot pressing.

[0017] In the aforementioned hot-pressing process, when the size of the intermediate porous polymer layer is equal to the size of the dense polymer film, after hot pressing, a 0.5–10 mm area at the edge of the multilayer film is locally sealed. This allows the upper and lower dense polymer films to fuse and encapsulate within this edge area. The sealing temperature is 180–280℃, the pressure is 0.2–2 MPa, and the time is 30–60 s. This procedure also applies to open-cell ferroelectric electret films prepared by hot pressing, 3D printing, etc., which do not possess waterproof and corrosion-resistant properties in their original state.

[0018] In both of these cases, thermoforming will not damage the porous structure of the central region of the electret.

[0019] Preferably, the polarization treatment is either high-voltage corona polarization or contact polarization. The high-voltage corona polarization treatment uses a voltage of ±10–20 kV, a polarization time of 2–10 minutes, a distance of 1–10 cm between the corona needle and the fluorinated porous film, and a contact polarization voltage of ±1–10 kV.

[0020] The present invention also provides a fluorine-containing ferroelectric electret prepared by the above preparation method, which has piezoelectric properties and is also waterproof and corrosion resistant.

[0021] Preferably, conductive electrodes may be provided on the upper and lower sides of the fluorine-containing ferroelectric electret, and the electrodes may be metal thin films, conductive fabrics, or conductive coatings.

[0022] Preferably, waterproof and corrosion-resistant fluorinated ferroelectric electrets are suitable for use in flexible wearable sensors, waterproof acoustic devices, self-powered energy harvesting systems, and electronic devices in high-humidity and high-salt environments.

[0023] Therefore, the present invention has the following beneficial effects:

[0024] (1) Improved waterproof and corrosion resistance: By forming a waterproof sealing layer at the edge of the ePTFE (expanded / porous polytetrafluoroethylene) based ferroelectric electret, the waterproof and corrosion resistance of the ferroelectric electret in humid and high-temperature environments is improved, solving the problem that existing ferroelectric electrets are prone to moisture absorption and charge loss.

[0025] (2) Maintaining electrical properties and piezoelectric response: By sealing the edge microporous structure with a sealing layer, the original porous structure is not damaged, and waterproof and corrosion-resistant properties are achieved while maintaining electrical properties and piezoelectric response.

[0026] (3) Avoid using additional media and improve stability: No additional media such as adhesives or silicone are required, thus avoiding the damping and thermal aging problems caused by additional media. ePTFE and FEP (perfluoroethylene propylene) fluoropolymer materials themselves have excellent high temperature resistance and chemical stability, enabling the ferroelectric electret film prepared by this invention to work stably in high temperature or corrosive environments.

[0027] (4) Strong compatibility of hot pressing process: The hot pressing packaging process in this invention has strong compatibility and can be used for local packaging of films of different shapes without significantly increasing the thickness of the device or affecting its flexibility.

[0028] (5) Simple process, suitable for mass production: The preparation process is simple and mature, requiring only conventional process steps such as hot pressing and corona polarization, without increasing material costs. The process is easy to control and suitable for large-scale production.

[0029] (6) Wide applicability and equipment compatibility: The prepared waterproof piezoelectric ePTFE film has wide applicability and can be applied to fields with high environmental adaptability requirements, such as flexible sensors, nano-power generation devices, and waterproof electroacoustic components, to meet the performance requirements of the corresponding applications. At the same time, the preparation method uses general-purpose manufacturing equipment, which lowers the technical application threshold.

[0030] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the waterproof and corrosion-resistant fluorine-containing ferroelectric electret in Embodiment 1 of the present invention;

[0032] Figure 2This is a flowchart illustrating the preparation process of the waterproof and corrosion-resistant fluorinated ferroelectric electret in Example 1 of the present invention.

[0033] Figure 3 This is a process flow diagram of the hot pressing method for the waterproof and corrosion-resistant fluorinated ferroelectric electret in Embodiment 1 of the present invention;

[0034] Figure 4 This is a process flow diagram of the edge sealing path of the waterproof and corrosion-resistant fluorine-containing ferroelectric electret in Embodiment 2 of the present invention;

[0035] Figure 5 The waterproof and corrosion-resistant ferroelectric electret prepared in Example 1 of this invention and the ferroelectric electret with the same parameters and opening, were subjected to different times in a distilled water environment. d 33 Comparison chart;

[0036] Figure 6 These are the areas before and after the edge sealing treatment of the waterproof and corrosion-resistant fluorine-containing ferroelectric electret in Embodiment 2 of the present invention. d 33 Value comparison chart;

[0037] Figure 7 The waterproof and corrosion-resistant ferroelectric electret prepared in Example 2 of this invention and the ferroelectric electret with the same parameters and open holes were subjected to different times in a salinity (45%) salt water environment. d 33 Value comparison chart;

[0038] Figure 8 This is an electron microscope image of the cross-sectional morphology of the sample prepared in Example 3 of this invention. Detailed Implementation

[0039] The technical solution of the present invention will be further described below through examples and embodiments.

[0040] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0041] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art. These other embodiments are also covered within the scope of protection of this invention.

[0042] Example 1

[0043] This embodiment provides a waterproof and corrosion-resistant fluorine-containing ferroelectric electret, such as... Figure 1As shown, a 4cm×cm square FEP film with a thickness of 15μm and an ePTFE film with a thickness of 3cm×3cm and a thickness of 15μm were selected as raw materials, and the three-layer film of FEP / ePTFE / FEP was stacked together. The preparation process is as follows. Figure 2 As shown.

[0044] The method for preparing waterproof and corrosion-resistant fluorinated ferroelectric electret provided in this embodiment specifically includes, as follows: Figure 3 The steps shown involve setting the target temperature of the hot press to 280°C. Once both the upper and lower plates reach 280°C, the three-layer FEP / ePTFE / FEP film is placed in the middle of the hot press and hot-pressed for 20 minutes. The FEP on both sides melts and bonds with the ePTFE. When the FEP is larger than the ePTFE, the two FEP layers at the edge can melt together under gravity or pressure. In general, placing a metal film at the edge can melt the upper and lower FEP layers together, sealing the open pores of the ePTFE, forming a dense barrier, and preventing water molecules and ions from penetrating the porous structure, thus obtaining a multi-layer structure.

[0045] Subsequently, a polarization voltage of +10kV was selected, and the multilayer structure formed by hot pressing was placed in the middle position of the high-voltage piezoelectric polarization device and polarized for 5 minutes. This ionized the air inside the porous film and separated the positive and negative charges in the pores, thus preparing a ferroelectric electret through corona polarization.

[0046] The hot-pressed ferroelectric electret and the untreated ferroelectric electret prepared in this embodiment were placed in 200ml of distilled water at a depth of 7cm for 5, 10, 15, 20, 25, and 30 minutes, respectively. Afterward, the samples were removed, and residual water droplets were absorbed with lint-free paper. Measurements were then taken. d 33 Values, test results as follows Figure 5 As shown, the sample after thermopressing in this embodiment d 33 The decay rate of the value was significantly lower than that of the untreated sample with openings, and the sample after heat-pressing and sealing was immersed in distilled water for a long time. d 33 The values ​​were significantly higher than those of the untreated sample with open pores. The same experiment was conducted in 200 ml of artificial seawater at a depth of 7 cm with salinities of 25, 35, and 45, and the corresponding experimental phenomena were also observed, indicating that the hot-press encapsulation treatment of the ferroelectric electret in this embodiment improved the stability and sustainability of the ferroelectric electret in humid and corrosive environments.

[0047] Example 2

[0048] This embodiment addresses the issue of treating a previously prepared porous ferroelectric electret that lacks waterproof and corrosion-resistant capabilities. By altering the treatment parameters, it acquires waterproof and corrosion-resistant properties without changing the ferroelectric electret itself. Taking a 2cm×2cm porous FEP / ePTFE / FEP ferroelectric electret as an example, the ePTFE film thickness is 10μm, and the FEP thickness is 15μm.

[0049] like Figure 4 As shown, the procedure includes the following steps: The sample is placed under a polarization device for corona polarization treatment. Since the selected sample parameters are basically the same as in Example 1, except that the edge is an open structure, the polarization method and parameters are the same as in Example 1. The open-hole ferroelectric electret is placed under a specific hot-pressing sealing mechanism, and a 2mm wide edge is locally sealed. Heating is performed at 200℃, 1-2MPa, for 32 seconds to form a continuous fused encapsulation layer of FEP / ePTFE / FEP at the edge, physically isolating moisture and other substances from entering. Measurements are taken of each region before and after the edge sealing treatment of the ferroelectric electret. d 33 Value changes as Figure 6 As shown, in all measurement areas, after hot pressing (the area with the dark diagonal lines), d 33 The value decreased only slightly compared to before treatment (lighter areas). This indicates that the edge-sealing hot-pressing treatment had only a slight effect on the various regions of the ferroelectric electret. d 33 The value will not be significantly affected. Comparative measurements were taken after the ferroelectric electret was placed in distilled water and artificial saline solutions with salinity levels of 25, 35, and 45, before and after edge treatment. d 33 The values, experimental and measurement methods are the same as in Example 1, so they will not be repeated here. This was done in artificial seawater with a maximum salinity of 45. d 33 Value decay pairs, for example Figure 7 As shown, the sealing treatment can significantly slow down the ferroelectric electret. d 33 The decay rate of the value is controlled to maintain the relatively stable piezoelectric properties of the material. Ferroelectric electrets under normal conditions are easily affected by artificial seawater. d 33 The value drops rapidly. Therefore, the above experiments show that the open-pore ferroelectric electret treated with edge hot pressing can be well retained in distilled water and gradient salt water environments. d 33 The value is high, and after treatment, the openings are only sealed by a dense fused layer, which effectively blocks the penetration of water molecules and ions, thereby optimizing the waterproof and corrosion-resistant performance of the ferroelectric electret. Moreover, the edge treatment range is small and will not affect the porous fiber network structure of ePTFE in the middle of the ferroelectric electret. Therefore, the waterproof and corrosion-resistant treatment will not have any other impact on the ferroelectric electret.

[0050] Example 3

[0051] This embodiment uses a 2cm×2cm FEP / ePTFE / FEP ferroelectric electret with openings as an example. The ePTFE film has a thickness of 10μm and a pore size of 3-5μm, while the FEP film has a thickness of 15μm. The left half of the sample cross-section in this embodiment is treated with waterproofing and corrosion protection using the method described in Example 2, while the right half is left untreated. The result... Figure 8 As shown, the left side is the encapsulated cross-section, illustrating that a closed cross-section can be obtained using the method provided by this invention, while the untreated part on the right side remains open.

[0052] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a waterproof and corrosion-resistant fluorine-containing ferroelectric electret, characterized in that, The fluorine-containing ferroelectric electret is prepared by edge-melting and encapsulating two dense polymer films and an intermediate layer, followed by polarization treatment. The intermediate layer is a single-layer structure of porous polymer film or a multi-layer alternating structure of porous polymer film and dense polymer film. Edge melting encapsulation is achieved through a hot pressing process, which includes the following steps: stacking the dense polymer films on both sides and the middle layer on a hot press, adjusting the hot pressing parameters to melt and bond the edges of the upper and lower dense polymer films to form a seal; The size of the intermediate layer is smaller than that of the dense polymer film. The intermediate layer is placed in the center between the dense polymer films. The edges of the dense polymer film overflowing around the intermediate layer are melt-sealed. The hot-pressing temperature is 250–300°C, the hot-pressing pressure is 0.5–5 MPa, and the hot-pressing time is 30–120 minutes. When the thickness of the intermediate layer is greater than 20μm, a metal plate frame is placed at the edge of the top dense polymer film during the hot pressing process. The thickness of the metal plate frame is the same as the thickness of the intermediate porous layer after hot pressing. The size of the intermediate layer is equal to the size of the dense polymer film. After hot pressing, the edge of the multilayer polymer film is partially sealed in a 0.5–10 mm area, so that the dense polymer films on the upper and lower sides are fused and sealed in this edge area. The sealing temperature is 180–280℃, the pressure is 0.2–2MPa, and the time is 30–60s.

2. The method for preparing a waterproof and corrosion-resistant fluorinated ferroelectric electret according to claim 1, characterized in that, The porous polymer film has an interconnected and interwoven polymer filament structure, a thickness of 1-200 micrometers, and an opening pore size of 0.5-20 micrometers. The porous polymer film is made of any one of expanded polytetrafluoroethylene, porous polytetrafluoroethylene, or porous perfluoroethylene propylene film. The dense polymer film is a fluoropolymer solid film layer.

3. The method for preparing a waterproof and corrosion-resistant fluorinated ferroelectric electret according to claim 1, characterized in that, The polarization treatment is either high-voltage corona polarization or contact polarization. The high-voltage corona polarization treatment uses a voltage of ±10–20 kV, a polarization time of 2–10 minutes, a distance of 1–10 cm between the corona needle and the fluorinated porous film, and a contact polarization voltage of ±1–10 kV.

4. A waterproof and corrosion-resistant fluorinated ferroelectric electret, characterized in that, The waterproof and corrosion-resistant fluorinated ferroelectric electret is prepared by the preparation method described in any one of claims 1-3.

5. A waterproof and corrosion-resistant fluorinated ferroelectric electret according to claim 4, characterized in that, The fluorine-containing ferroelectric electret is provided with conductive electrodes on its upper and lower sides, and the electrodes are metal thin films, conductive fabrics or conductive coatings.

6. The waterproof and corrosion-resistant fluorinated ferroelectric electret according to claim 4, characterized in that, Waterproof and corrosion-resistant fluorinated ferroelectric electrets are suitable for use in flexible wearable sensors, waterproof acoustic devices, self-powered energy harvesting systems, and electronic devices in high-humidity and high-salt environments.

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